FAMILY TORN ASUNDER: James Manzella with his fiancée, Jamie Taccetta, and her daughter Kaitlyn.

Instead of planning their wedding, he’s planning her funeral.

A Long Island electrician — who was to wed his longtime girlfriend this fall — was the first person to discover the gruesome Haven Pharmacy slaying, stumbling onto the blood-soaked scene after he grew worried that she took too long to pick up a prescription.

A devastated James Manzella described yesterday how his worst fears were realized when he entered the drugstore to find his soul mate, Jamie Taccetta, lying dead in a pool of blood, one of four people slaughtered by a crazed drug-seeking gunman.

“This woman was the love of my life,” Manzella told The Post yesterday.

All he can think about now, he said, is “if I could have done something” to save her from being murdered as he sat in their car waiting for her to pick up thyroid medication.

“Can you imagine sitting there in air conditioning, listening to music, and then the next minute she’s gone?”

Taccetta, 33, a physical therapist who has two daughters — Miranda, 16, and Kaitlyn, 5 — first met Manzella at her job while his mother was receiving treatment.

She had recently moved in with him at his Farmingville home after Manzella’s mom died this past winter — and he proposed to her last year.

“She was so happy where she was in life,” said Taccetta’s dad, Ralph. “She was going to get married, and she was so excited about it. You could tell they really loved each other.”

Manzella and Taccetta began planning their wedding, set for Oct. 15, and bought her wedding dress.

On Sunday morning, Manzella drove Taccetta to the pharmacy on Southaven Avenue in Medford.

He parked the car, she got out to go inside to collect her prescription, and he sat and relaxed as he listened to a Nick Cannon song on his stereo.

Some time passed, and Manzella began wondering what was taking Taccetta so long, he said yesterday.

Then, Manzella spotted an odd-looking bearded man walking out of the pharmacy, wearing sunglasses, a white baseball hat and a backpack over a hoodie sweatshirt.

“I saw the guy walk out, and I thought something was weird,” Manzella said. “So I turned the car off to check on her.”

“I walked to the front” of the pharmacy, then went inside, even as the mysterious man walked down the block, Manzella recalled.

Manzella saw Taccetta, shot in the head from behind — along with Bryon Sheffield, 71, another customer who had walked in and been gunned down by the same killer who had also slain two pharmacy workers so he could rob the shop of painkillers.

“It was horrible, man. It was something I’ll never forget,” Manzella said. “I’ll never forget what I saw. It’s stuck in me.”

Manzella’s frantic 911 call to cops brought authorities to the scene.

Yesterday, Manzella said he was haunted by the fact that he had seen his fiancée’s killer moments after the murders.

“I could see the guy clearly, and now I think back if I could have done something,” Manzella said. “That’s what messes with you. We were two average people trying to do the right thing, and this is what happens. It’s not fair.”

At his home in Shirley, where his daughter’s wedding dress hung downstairs, Taccetta’s father phoned his wife, who was on vacation in Jamaica, to tell her to fly back immediately — without disclosing that Jamie was dead.

“She’s doesn’t even know yet,” Ralph Taccetta said yesterday. “She’s coming home tonight, and I’m going to have to tell her. We just said there’s a family emergency and you need to come home.”